We had the idea to build aerodynamically formed fins by laying carbon fiber laminate on a preformed fincore. a we were not sure wether a core of XPS(extruded polystyrene) material – which is very cheap and easy to deal with – would have the necessary tensile strength to support the laminates caron layers on both sides. so we decided to do a test run by building a shaft-element with only two fins and build up load until destruction …
- Fin height : 120mm
- Root chord: 160mm
- Tip chord: 80mm
- Sweep length: 40mm
- Shaft diameter : 56mm
The first challenge was to find a way, how to preform a very exact even if fragile fincore. therefore we built a special jig that allowed us to perform milling operations on all three sides of the XPS-element.
The raw form of the XPS part was milled out of al XPS board (you can buy it at anny regular hardware store for insulation purpouse) and a 1.5mm groove was cut around its contour.
The groove fixes the raw cores postition in the jig and at first the rounding to fit the shaft was made (later on, when the composite part is finished a “lipping” will be glued into the gap):
After that both deck-sides were milled with a ball-end-tool.
And here is the finished XPS fincore:
And here on shaft:
Then we had to build another jig for the lamination process (here again the groove along the fins coutour helps to keep everything in place!):
A view with fincores inside:
Sadly we dont have pictures from the laminatrion process. On each side we put on three layers of carbon fiber (45/45 cloth; UD layer; 90/90 cloth). After curing in our self-made oven we released it from the jig: first we tried to bend it by hands but it felt very rigid and tiff; after that we put the testfins between two charis and started to load weight on the center shaft. even at a load of 300N it seemed stiff and tough.
After that we wanted to see how far we could go, and so we produced the folowing video deliberately destroying our prototype:
we wantet know where exactly the weakest point of our construction is. as you can see the testfins took up to 60kg (first person steping on it) but failed then at 65kg (second person). The fin core suffered delamination – but without the “lipping” that should be glued into the groove along the fins coutour (which should give it cosiderably more strength) and at a much higher load than we expected.